Applications in which a forced airflow is used with a premix burner require a high-pressure fuel delivery system in order to overcome the high air pressures (generally from 2˜15 psi) in the burner. In the present systems, fuel is metered in a binary fashion using several solenoid valves (see U.S. Pat. Nos. 6,135,063 and 4,462,342, for example) or with electronic controls utilizing several sensors and valves (see U.S. Pat. No. 5,685,707, for example).
A typical known premix burner layout includes main fuel and air passages which merge at a burner inlet and are fed into the burner combustion chamber where the mixed fuel and air are ignited. Water for producing steam is introduced into an inlet at one end of the chamber and moves along an inner surface of the chamber toward an outlet at an opposite end of the chamber. Combustion occurs centrally within the chamber and the heat generated changes the water to steam. U.S. Pat. No. 4,211,071 discloses such a system. Systems of this type having fuel metered in the known ways have one or more of the following drawbacks: (1) the burner's output is changed in a step progression, with each step depending on the size and quantity of valves opened to regulate the fuel/air mixture; (2) fuel flow is often not related to the airflow through the burner; (3) to have a variety of firing rates, either a fuel flow control system including several fuel valves is needed, or a complicated variable valve system is needed, with either system adding great expense while decreasing overall reliability; and requiring a high amount of piping which makes them undesirable for mobile application.
It is desirable then to overcome the cost and complexity of current fuel systems for premix burners of direct-fired steam generators, while improving the firing rate control.